Brain monoamines and antidepressant-like responses in MRL/MpJ versus C57BL/6J mice

Abstract

The MRL/MpJ mouse demonstrates enhanced wound healing and tissue regeneration and increased neurotrophic mobilization to chronic antidepressant drug treatments. This study compared brain monoamine systems between MRL/MpJ and C57BL/6J mice as a potential basis for strain differences after chronic antidepressant treatment. MRL/MpJ mice had significantly higher tissue levels of serotonin and dopamine in multiple brain regions. Microdialysis studies demonstrated that baseline levels of extracellular serotonin did not differ between strains. However, acute administration of the selective serotonin reuptake inhibitor citalopram produced an increase in extracellular serotonin in the ventral hippocampus of MRL/MpJ mice that was twice as large as achieved in C57BL/6J mice. The greater effects in MRL/MpJ mice on 5-HT levels were not maintained after local perfusion of citalopram, suggesting that mechanisms outside of the hippocampus were responsible for the greater effect of citalopram after systemic injection. The density of serotonin and norepinephrine transporters in the hippocampus was significantly higher in MRL/MpJ mice. In addition, the expression of 5-HT(1A) mRNA was lower in the hippocampus, 5-HT(1B) mRNA was higher in the hippocampus and brainstem and SERT mRNA was higher in the brain stem of MRL/MpJ mice. The exaggerated neurotransmitter release in MRL/MpJ mice was accompanied by reduced baseline immobility in the tail suspension test and a greater reduction of immobility produced by citalopram or the tricyclic antidepressant desipramine. These data suggest that differences in the response to acute and chronic antidepressant treatments between the two strains could be attributed to differences in serotonin or catecholamine transmission.

Fig. 1

The effects of citalopram on extracellular 5-HT levels in the ventral hippocampus of MRL and B6 mice. A) Systemic injection of citalopram (20 mg/kg; at arrow) increased extracellular 5-HT levels in both strains, MRL mice showed a greater increase than B6 mice at all time-points following citalopram administration. There were no differences between mouse strains in baseline 5-HT levels (120-180 min) collected prior to injection (B6: filled square, n = 5; MRL: open square, n = 8). B) Citalopram was perfused directly into the hippocampus (1 mM; starting at the arrow) using a liquid switch. Citalopram increased extracellular 5-HT levels in both strains (after 220 min). Levels of 5-HT were higher in MRL than B6 mice at two time points (240 and 280 min). There were no differences in baseline (120-180 min) 5-HT levels between mouse strains before perfusion (B6: filled square, n = 7; MRL: open square, n = 11). Asterisks indicate significant differences between strains: * p < 0.05; ** p < 0.01.

Fig. 2

Comparison of SERT and NET binding between B6 and MRL mice. The density of SERT was measured using [³H]-paroxetine (Panel A) and NET was measured using [³H]-nisoxetine (Panel B) at concentrations producing a maximum level of specific binding in B6 (black bars; n = 4) and MRL (open bars; n = 4) mice. Bars represent mean specific binding (fmoles/mg protein) ± SEM. Asterisks indicate significant differences between groups according to two-tailed Student’s t-test (** p < 0.005).

Fig 3

Comparison of mRNA expression of SERT, 5-HT_1A and 5-HT_1B receptors in the hippocampus and brainstem between B6 and MRL mice. A) MRL mice (white bars, n = 7-8) showed increased 5-HT_1B gene expression in the hippocampus and brainstem, while showing decreased 5-HT_1A mRNA expression in the hippocampus in comparison to B6 mice (black bars, n=6-8 mice per group). Bars represent mean fold change normalized to B6 ± SEM. Asterisks indicate significant differences between groups according to two-tailed Student’s t-test (* p<0.05, ** p<0.01). B) MRL mice (white bars, n = 7-16) showed increases in SERT mRNA expression in the brainstem compared to B6 mice (black bars, n=7-16). Bars represent mean fold change normalized to B6 mice ± SEM. Asterisks indicate significant differences between groups according to two-tailed Student’s t-test (* p < 0.05, ** p < 0.01, *** p < 0.001).

Fig. 4

Effect of citalopram in the TST and on locomotor activity in B6 and MRL mice. Saline or citalopram (CIT: 10 mg/kg, striped bar; 20 mg/kg, black bar) was administered 30 min prior to testing. N = 9-10 mice/group. (A) Vertical bars represent the mean amount of time (sec) spent immobile during the 6-min test ± SEM. (B) The magnitude of change (%) in immobility for each dose of drug compared with absolute saline values. (C) Effect of citalopram on locomotor activity in B6 and MRL mice. Vertical bars represent the mean distance traveled (cm) during the 30 min period ± SEM. (D) The magnitude of change (%) in locomotion for each dose of drug compared with absolute saline values. Asterisks indicate groups that differed significantly from corresponding controls (*p < 0.05, **p < 0.005) according to Dunnett’s post-hoc analysis.

Fig. 5

Effect of desipramine in the TST and on locomotor activity in B6 and MRL mice. Saline or desipramine (DMI: 10 mg/kg, striped bar; 20 mg/kg, black bar) was administered 30 min prior to testing. N = 10-20 mice/group. (A) Values represent the mean amount of time (sec) spent immobile during the 6-min test. (B) Values represent the magnitude of change (%) in immobility for each dose of drug. Asterisk indicates groups that differed significantly from control (**p < 0.005) according to Dunnett’s post-hoc analysis. (C) Effect of desipramine on locomotor activity in B6 and MRL mice. Values represent the mean distance traveled (cm) during the 30 min period. (D) The magnitude of change (%) in locomotion for each dose of drug compared with absolute saline values. Asterisks indicate groups that differed significantly from corresponding controls (* p < 0.05, ** p < 0.005) according to Dunnett’s post-hoc analysis.